(1) Field of the Invention
The present invention relates generally to the regeneration of a spent scrubbing solution containing a polyvalent metal chelate subsequent to its use in removal of hydrogen sulfide from a sour, gaseous stream.
(2) Description of the Prior Art
The use of an aqueous chelated polyvalent metal catalyst solution for removing hydrogen sulfide from a sour, gas stream is well known in the art. The chelated polyvalent metal aqueous solution upon contact with the hydrogen sulfide-containing gas stream effects oxidation of the hydrogen sulfide to elemental sulfur and at the same time the polyvalent metal chelate is reduced to a lower valence state. Most frequently the polyvalent metal utilized is iron. Regeneration of the chelated polyvalent metal solution by the oxidation of the ferrous chelate to the ferric chelate is most generally accomplished by contacting the solution with an oxygen-containing gas. U.S. Pat. No. 4,622,212 to McManus et al provides a representative listing of United States patents illustrating processes for removal of hydrogen sulfide from a sour gaseous stream using a polyvalent metal chelate. In McManus et al, excessive degradation during regeneration of the chelating agent is prevented by incorporation of a stabilizing agent such as an alkaline thiosulfate in the polyvalent metal chelate scrubbing solution. This patent is hereby incorporated by reference as providing a particularly thorough discussion of the early prior art showing regeneration of spent polyvalent metal chelate with an oxygen containing gas. A more recent patent in this area is U.S. Pat. No. 4,696,802 to Bedell.
In U.S. Pat. No. 4,455,287 to Primack et al, a method of stabilizing a chelated polyvalent metal, utilized in a scrubbing solution for removing hydrogen sulfide from a gas stream, is disclosed as the use of a broad spectrum biocide. This prevents degradation of the chelated polyvalent metal caused by contamination of the aqueous composition with microorganisms such as bacteria.
In U.S. Pat. No. 4,532,118 to Tajiri et al, the degeneration of a chelated polyvalent metal utilized in a scrubbing solution for removal of hydrogen sulfide from a gas stream is referred to as causing the formation of ferrous sulfide which when mixed with the free sulfur product causes a darkening in color of the sulfur, thereby impairing the commercial value thereof. The means suggested by Tajiri et al of reducing degradation of the polyvalent metal chelate is to adjust the mole ratio of ferric ion to total iron ions in the scrubbing solution in the range of 0.6 to 0.9.
Regeneration of spent scrubbing solutions utilizing an electrochemical cell is also known in the art. In U.S. Pat. No. 4,126,529 to DeBerry, the spent ferric chelate containing scrubbing solution utilized to remove oxides of nitrogen and sulfur from flue gases is regenerated by passing the solution through the cathode compartment of an electrochemical cell. The regeneration process involves the removal of the sulfate ions from the scrubbing solution through the ion transfer membrane and the reduction of the non-reactive ferric chelate to the reactive ferrous chelate. In U.S. Pat. No. 4,076,793 to Nikolai a method is disclosed for removing sulfur dioxide from a gas stream utilizing an aqueous slurry of manganous hydroxide as an absorbent. Manganous sulfate is regenerated to manganous hydroxide by aqueous phase electrochemical precipitation from the spent scrubbing solution. In U.S. Pat. No. 4,041,129 to Foster et al, a process is disclosed for removal of acidic gases from hydrocarbon streams in which regeneration of the scrubbing solution is disclosed as utilizing an electrolytic cell. In this process an aqueous sodium hydroxide solution is utilized to scrub the acidic gases. The rich, scrubbing solution is thereafter reacted with aqueous sulfuric acid to liberate the acidic gases such as hydrogen sulfide and the resulting aqueous sodium sulfate solution is converted electrolytically to sodium hydroxide at the cathode of an electrochemical cell.
Regeneration of spent hydrogen sulfide scrubbing solutions utilizing a fuel cell is disclosed in U.S. Pat. No. 4,320,180 to Nozaki and U.S. Pat. No. 4,436,713 and U.S. Pat. No. 4,436,711 both to Olson. The Olson patents indicate that degradation of the polyvalent metal chelate may be caused by or enhanced by regeneration of the spent scrubbing solution with oxygen or an oxygen-containing gas and that regeneration utilizing a fuel cell avoids this problem.
Regeneration of a hydrogen sulfide spent gas scrubbing solution containing a polyvalent metal chelate by the use of an electrolytic cell is disclosed in U.S. Pat. No. 4,643,886 to Chang et al and in the Olson U.S. Pat. Nos. 4,436,712 and '714 and U.S. Pat. Nos. 4,443,423 and '424. Each of these patents teaches reduced degradation of the polyvalent metal chelate during regeneration of the chelate in an electrolytic cell as compared to regeneration using oxygen or an oxygen-containing gas.
In none of the prior art references in which a polyvalent metal chelate is regenerated utilizing an electrolytic cell or a fuel cell is the electrical potential imposed upon the electrolytic cell controlled such that the potential of the cell is maintained at a value lower than the oxidation potential of the polyvalent metal chelate. The applicant is therefore the first to discover that all degradation of a polyvalent metal chelate can be avoided during regeneration of a polyvalent metal chelate from a lower valence state to a higher valence state in the anode compartment of an electrolytic cell simply by maintaining the anode potential below the oxidation potential of the chelate portion of the polyvalent metal chelate being regenerated.